I'm not a geneticist but I am majoring in genetics, so I can give you a simplified explanation.

In your family, it is likely you have a case of something called incomplete dominance. You can do a google search for the definition but an example of this would be if you crossed a red flower with a white flower and ended up with pink flowers. In this example, the red and white genes sort of combine to make a new color.

The same goes for hair. Your dad gave you a curly hair gene, and your mom gave you a straight hair gene. When these two combine, you get something that falls in between, in your case wavy hair.

Unfortunately, the interaction of genes that determine how curly your hair is is a complicated process, which I don't have time to explain here. It is possible that with age/changing hormones your hair will get more curly, but at this point you can't predict anything.